Because constituents will often fight such an attempt. For every covenant that's annoying you, there are people in the vicinity that like it.

People generally like setbacks. It prevents their neighbors from being too close, or the street getting crowded in. Property owners might want to develop along the edges, but their neighbors may not want them to.

So clearing these covenants out wholesale is going to be a fight. And going through the legislative process for a single property is really inefficient.